add fileOpenDialog and enable loading of urdf from GUI

(will add .bullet file support soon)
Uses native Windows (getFileOpenFileName) and Mac OSX NSOpenPanel,
on Linux using pipe popen to zenity)

Demos3/GpuDemos/gwenUserInterface.h

@@ -5,7 +5,7 @@ struct GwenInternalData;
 
 typedef void (*b3ComboBoxCallback) (int combobox, const char* item);
 typedef void (*b3ToggleButtonCallback)(int button, int state);
-
+typedef void (*b3FileOpenCallback)();
 
 
 class GwenUserInterface
@@ -40,6 +40,8 @@ class GwenUserInterface
 		
 		void	setStatusBarMessage(const char* message, bool isLeft=true);
 
+        void    registerFileOpenCallback(b3FileOpenCallback callback);
+    
 		GwenInternalData* getInternalData()
 		{
 			return m_data;